240 On the Evolution of the Vertebrata, [ March, 
mentioned are present in excess, and others are added, as the 
loss of ossification of a part of the skull, atmost total obliteration 
of the hyoid and scapular arches, and the semi-notochordal condi- 
tion of the vertebral column, etc. 
The Physoclysti nearest the Physostomi have abdominal ven- 
tral fins, and belong to several orders. It is such types as these 
that may be supposed to have been derived directly from Holos- 
tean ancestors, They appear in the Cretaceous period (Derceti- 
dz), along with the types that connect with the Physostomi 
(Haplomi). Intermediate forms between these and typical Phy- 
soclysti occur in the Eocene (Trichophanes, Erismatopterus), 
showing several lines of descent. The Dercetidæ belong appa- 
rently to the order Hemibranchi, while the Eocene genera named 
belong apparently to the Aphododiride, the immediate ancestor 
of the highest Physoclysti, the Percomorphi, The order Hemi- 
branchi is a series of much interest. Its members lose the mem- 
brane of their dorsal spinous fin (Gasterosteidæ), and then the fin 
itself (Fistularia, Pegasus). The branchial apparatus has under- 
gone, as in the eels, successive deössification (by retardation), and 
this in direct relation to the degree with which the body comes 
to be protected by bony shields, reaching the greatest defect in the 
Amphisilida. One more downward step is seen in the next suc- 
ceeding order of the Lophobranchii. The branchial hyoid appa- 
ratus is reduced to a few cartilaginous pieces and the branchial 
fringes are much reduced in size. In the Hippocampide the 
caudal fin disappears and the tail becomes a prehensile organ by 
the aid of which the species lead a sedentary life. The mouth 
is much contracted and becomes the anterior orifice of a sucto- ` 
rial tube. This is a second line of unmistakable degeneracy 
among true fishes. 
_ The Physoclysti with pectoral ventral fins present us with per- 
haps ten important ordinal or subordinal divisions. Until the 
palzontology of this series is better known, we shall have diffi- 
culty in constructing phylogenies. Some of the lines may, how- 
ever, be made out. The accompanying diagram will assist in un- 
derstanding them. 
_ The Anacanthini present a general weakening of the organiza- 
tion in the less firmness of the osseous tissue and the frequent 
~ reduction in the size and character of the fins. The caudal ver- 
~ tebree are of the protocercal type. As this group does not appear 
